Electronic article surveillance (“EAS”) systems are commonly used in retail stores and other settings to prevent the unauthorized removal of goods from a protected area. Typically, a detection system is configured at an exit from the protected area, which comprises one or more transmitters and antennas (“pedestals”) capable of generating an electromagnetic field across the exit, known as the “interrogation zone.” Articles to be protected are tagged with an EAS marker that, when active, generates a response signal when passed through this interrogation zone. An antenna and receiver in the same or another “pedestal” detects this response signal and generates an alarm.
Metal detection systems are also useful in detecting the unauthorized removal of metal items from a protected area. While there exist many metal detection systems, there has been no successful attempt to combine an EAS detection system with a metal detection system. While others have provided a metal detection system adjacent to an EAS system, no one has provided any mechanism for the increased efficiency and cost reduction of actually combining the two systems into one.
Part of the difficulty in combining both systems into one is the problems that arise due to interference from the transmission signals of other, adjacent EAS transmitters. If the metal detector is integrated with an acoustomagnetic (“AM”) EAS system and both use the same transmitter electronics and tuned antenna coils to transmit both EAS and metal detection signals, an adjacent EAS transmitter will interfere with the metal detection receiver. Even if the metal detection frequency is different from the EAS frequency, the sidebands of the adjacent EAS transmitter will have a considerable impact in the metal detection receiver. The metal detection frequency cannot vary much from the EAS frequency because the metal detection transmission amplitude will be greatly reduced. This is due to the fact that the systems' antennas are tuned to the EAS transmit frequency.
Therefore, what is needed is an integrated EAS/metal detection system that reduces the impact of interference that may be caused by an adjacent EAS transmitter.